Pyridonecarboxylic acid and pharmacologically acceptable salts thereof have been known to be extremely superior synthetic antibacterial agents. However, since said pyridonecarboxylic acid has carboxylic acid and the dihydropyridine skeleton in a molecule, forming a zwitterion, it shows strikingly low solubility in water at a physiological pH, i.e., neutral range. This imposes a problem that an aqueous solution containing pyridonecarboxylic acid or a pharmacologically acceptable salt thereof cannot be formulated into a pharmaceutical preparation at a neutral pH.
Hardly soluble pyridonecarboxylic acid and a pharmacologically acceptable salt thereof are solubilized by, for example, adding an inorganic acid to pyridonecarboxylic lactate (U.S. Pat. No. 4,705,789, U.S. Pat. No. 4,808,583, U.S. Pat. No. 4,808,585), or adding a base in excess to pyridonecarboxylic acid (U.S. Pat. No. 4,772,605), or adding a metal compound comprising aluminum, magnesium or zinc to pyridonecarboxylic acid or a salt thereof (Japanese Patent Unexamined Publication No. 188626/1988). The aqueous preparations thus obtained are associated with variations in pH which are caused by the solubilizer added, and the toxicity of the solubilizer itself which may cause a local irritation and the like, possibly leading to unexpected disorders.
Thus, there has not been provided an aqueous preparation containing solubilized pyridonecarboxylic acid, which is safe and useful at a physiological pH, i.e., about neutral pH.